Laminated films are widely used for packaging applications. The combinations of coating layers and substrates can be varied, depending on the intended application's functional requirement. The materials selected for each of the layers may be adjusted depending on whether physical properties such as thermal stability, barrier properties, abuse resistance, sealing, etc, or package aesthetics, such as high gloss, transparency, printing, etc., are more important. Further the materials may need to be adjusted to ensure compatibility between the materials.
A significant part of the market uses glossy films as external layers for laminated packages in order to promote either the appearance of the product or the packaging printing and therefore gamer the attention of end user at shelves. In addition to high gloss, such films must possess adequate stiffness for the aesthetics as well as functionality of the package.
Current films touted for use in high gloss packages typically use polyethylene terephthalate (PET), biaxially oriented polypropylene (BOPP) and cast polypropylene (PP) as the external layer or layers. These films are generally produced using cast process and can be submitted to an orientation process in order to increase stiffness and reduce thickness. The demand on these films is increasing year after year, with some restrictions on supply already occurring amongst laminated films producers.
Stiff, glossy and transparent films are especially appreciated for lamination purposes since they will be printed and used as an external layer in a laminated structure. Polyethylene films have small participation at the premium segment of this market due to its inherent characteristics of low stiffness, low gloss and high haze when compared to incumbent materials PET, cast PP and BOPP.
The use of high molecular weight polyethylene materials resins in cast film extrusion processes has recently been considered for such applications, mainly looking at the different film properties cast stretch films can obtain using those materials. Compared to blown film processes, the range of cooling rates of cast film extrusion process is broader and can be more easily adjusted according to the final film property desired. These process features are known in the art (see, for example, Billham, M.; Clarke, A. H.; Garrett, G.; McNally, G. M.; Murphy, W. R., The Effect of Extrusion Processing Conditions on the Properties of Blown and Cast Polyolefin Packaging Films, Dev. Chem. Eng. Mineral Process 1, pp. 137-146, 2003; or Giles, H. F. Jr.; Wagner, J. R. Jr.; Mount, E. Extrusion—The Definitive Processing Guide and Handbook, William Andrew Publishing/Plastics Design Library, 2005).
Polyethylene can be divided into high-density (HDPE, density 0.941 g/cm3 or higher), medium-density (MDPE density from 0.926 to 0.940 g/cm3), and low-density (LDPE, density from 0.910 to 0.925 g/cm3). See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials. Molecular weight of the polymer, which can be expressed as averages values (Mn, Mw, Mz), is correlated to the polymers melt index as determined according to ASTM D 1238 (2.16 kg, 190° C.).
In Wen Halle's article, “Opportunities for a New Metallocene Polyethylene in Shrink & Stretch Film Applications”, AMI Stretch & Shrink Films 2009 Oct. 26-27, 2009 Atlanta, Ga., advantages of using materials with low MI (that is, high molecular weight) at high strain levels, are taught, however those materials are generally in the density range of 0.910-0.940 g/cm3 depending on the functional layer they are assigned—cling, structural, release. US 007601409B2 and WO 2009/110887 describe increasing the strain hardening behavior of stretch film by adding a propylene based copolymer or mLLDPE materials that are stiffer when stretched.
US005916692 A discusses the use of LDPE materials in stretch films and the improvement in stiffness under high strain however the density range disclosed is from 0.915 to 0.935 g/cm3.
The use of HDPE type of materials for stretch films applications is very limited, as these materials have low toughness and tend to easily split once stretched. HDPE films are used when stiffness is the main requirement of the final application and the stiffness can be increased through film orientation after the extrusion.
US 20050197459 teaches the use of uniaxial machine direction orientation (MDO) process for enhancing the optical properties of blown HDPE films. This two step process, however, adds cost to the production chain and makes it non-competitive for lamination purposes.
The production of cast films may be used to generate films with good optical properties (high gloss and low haze) due to the combination of fast cooling and flat surfaces (chill roll), which reduces internal haze. On the other hand these films are generally not stiff enough to deliver the same performance as cast PP, BOPP or PET based films, even when HDPE materials are used.
Accordingly, there is still the need for polyethylene films having the combination of high stiffness and good optics. It has been found that the combination of resins with low MI in the core and high MI in the external layer (s), each with a density above 0.940 g/cm3 for use in a cast process may produce PE films with excellent performance, delivering high stiffness, high gloss levels and very low haze. Thus, present invention relates to a multilayer cast film comprising a first layer comprising a linear polyethylene having a density greater than 0.94 g/cm3 and a melt index less than or equal to 1.3 g/10 min together with a second layer comprising a linear polyethylene having a density greater than 0.94 g/cm3 and a melt index greater than or equal to 2.0 g/10 min. The second layer is an external layer of the film.